This invention relates to a device for detecting coincident focusing points and more particularly to a device for detecting coincident focusing points for recording an image of the object.
In a conventional detecting device of this type, it is well known to employ a system for detecting the coincidence of dual images, wherein two optical paths are provided, one of them being a fixed optical path, and the other being a movable light path in which information on the distribution of intensity of light may be changed. Each of the incoming light begins from the two light paths is received by two light receiving devices comprised of a plurality of light receiving elements, and a coincidence of focusing points is made when the distributions of light intensity in the two light receiving devices are coincident. As a method for detecting the time when the distributions of light intensity are coincident with each other the difference between the two light receiving devices is calculated to provide a point showing that all the calculated values become zero. However, in a practical example, due to a difference in sensitivity between each of the elements and a difference in efficiencies in the light path, it is a rare situation that all the difference values become zero. Thus, it was proposed to use a method in which the sum of the absolute values of differences between the outputs of each of the corresponding elements becomes a minimum value. In order to get a minimum value in accordance with this method, the movable incoming light is scanned over the full operating range and thereby it is required to determine the minimum value. The minimum value may be varied in reference to a level of light intensity at the circumference of the object to be photographed and thus it is not possible to get a minimum value by the coincidence of a specified value. It is essential to provide the scanning over the entire operating range. Even if such a scanning is performed, an accuracy is also restricted due to a difference in sensitivities of each of the elements and a difference in the light paths.
On the other hand, as a method for decreasing the influence of the level in the intensity of light at the circumference of the object to be photographed, it has been proposed to get the minimum value by a ratio on the basis of the principle in which a ratio of the outputs between the corresponding elements of the two light receiving devices equals to 1 resulting in a coincident focusing point. Since this proposed method may utilize a ratio between the light receiving devices, there still remains such effects as a difference between the sensitivities of each of the elements and a difference of efficiencies in the light paths.